Projections from the raphe nuclei to the phrenic motor nucleus in the cat

The purpose of this study was to identify the projections from the brain to the phrenic motor nucleus in the cat by employing a retrogradely-transported fluorescent dye. Propidium iodide was iontophoresed into the phrenic motor nucleus which is located in the fourth, fifth and sixth segments of the cervical spinal cord. Retrogradely-labeled cell bodies were found in the brainstem within recognized respiratory areas. In addition, retrogradely-labeled cell bodies were found within the raphe nuclei, nuclear areas not previously associated with respiratory control. The results of this study suggest that the raphe nuclei may play a role in the central regulation of breathing.